Personal cleansing with mild surface-active cleansing preparations has become a focus of great interest. Many people wash and scrub their skin with various surface-active preparations frequently. Ideal cleansers should be cost effective, cleanse gently and rinse well. Most synthetic toilet bars (synbars) fall short in this respect.
The synthetic detergents (detergent surfactants) which are sufficiently inexpensive to be considered for making bars and which have the requisite lather and cleansing power, are all far more soluble than the C.sub.12 -C.sub.18 fatty soaps which are used for bars. (The subscripts in C.sub.12 and C.sub.18 denote the number of carbon atoms in the acyl chains.) Furthermore, none of the synthetic products as yet matches the low cost of soap when compared on a 100% active ingredient basis. Still another difficulty with the synthetics is that they do not feel like soap when used for bathing. The familiar subjective sensation of slipperiness is somewhat modified in the case of most synthetics, and it persists even after prolonged rinsing. Soap feels the same way in very soft water, but in water of average hardness the slipperiness is rapidly quenched, presumably by the formation of lime soaps. In spite of all these difficulties, several toilet bars based on synthetic detergents are currently being marketed. Their chief advantages lie in their ability to lather profusely in hard water and in the fact that they do not form scum or deposits of dirt on the basin or bathtub walls. The best synthetic bars now being marketed cost considerably more than toilet soap bars.
A key mild synthetic surfactant is alkyl glyceryl ether sulfonate. See U.S. Pat. Nos.: 2,094,489, Hulter, issued Sep. 28, 1937; 2,427,576, Smith, issued Sep. 16, 1947; 2,427,577, Smith, issued Sep. 16, 1947; 2,988,511, Mills et al., issued Jun, 13, 1961; 2,989,547, Whyte, issued Jun. 20, 1961; 2,999,068, Pilcher et al., issued Sep. 5, 1961; and 3,024,273, Whyte et al., issued Mar. 6, 1962, all of said patents being incorporated herein by reference. It is noted that surfactant mildness can be measured by a skin barrier destruction test which is used to assess the irritancy potential of surfactants. In this test the milder the surfactant, the lesser the skin barrier is destroyed. Skin barrier destruction is measured by the relative amount of radio-labeled water (3H-H20) which passes from the test solution through the skin epidermis into the physiological buffer contained in the diffusate chamber. This test is described by T. J. Franz in the J. Invest. Dermatol., 1975, 64, pp. 190-195; in U.S. Pat. No. 4,673,525, Small et al., issued Jun. 16, 1987; and in copending U.S. patent application Ser. No. 294,832, filed Jan. 9, 1989, all of which are incorporated herein by reference. The patent and application disclose mild alkyl glyceryl ether sulfonate surfactant based synbars comprising a "Standard" alkyl glyceryl ether sulfonate mixture.
U.S. Pat. No. 4,180,470, Tokosh et al., issued Dec. 25, 1979, and incorporated herein by reference, discloses a method for making improved acyl isethionate detergent bars with 2-6% of sodium alkoxy hydroxy propane sulfonate (a synonym for alkyl glyceryl ether sulfonate) with alkyl chains of from 8 to 22 carbon atoms in conjunction with a small amount of sodium chloride. Several alkyl glyceryl ether sulfonates are disclosed and C.sub.18 alkyl glyceryl ether sulfonate at 5% is used in an example. The added alkyl glyceryl ether sulfonate and salt are minor ingredients used with the primary acyl isethionate detergent surfactant to improve bar wear rate without adversely affecting lathering characteristics. No rinse advantage is suggested for adding any alkyl glyceryl ether sulfonate to the bars.
Major drawbacks of most mild synthetic detergent surfactant toilet bars are poor lather and/or poor rinse feel. The use of high sudsing anionic detergent surfactants can yield acceptable lather volume. Unfortunately, the highest sudsing anionic surfactants are, in fact, poor in skin "patch test" mildness. While the moderately mild sodium lauryl glyceryl ether sulfonate (C.sub.12 AGS), is relatively good in lather potential, C.sub.16 -C.sub.18 AGS is relatively poor in lather. It will be appreciated that mildness, lather, and rinsability make surfactant selection a delicate balancing act. Thus, it will be appreciated that rather stringent requirements for mild synbars limit the choice of surface-active agents and final formulations represent some degree of compromise. Mildness is often obtained at the expense of effective cleansing, lathering and rinsing or vice versa. Needless to say, a superior synbar with good lather potential and good rinsability is hard to formulate.